People with poor posture tend to have weakness of certain muscles.  Dr. Vladimir Janda described an Upper Cross Syndrome with lower trapezius weakness associated with a flattened cervical spine, forward head posture, protracted and elevated shoulders, and an increased thoracic kyphosis.

Muscles require normal unrestricted joint mobility to activate more efficiently.  People who present with this Upper Cross Syndrome posture do not have optimal activation of the lower trapezius muscles in order to stabilize the shoulder blades and draw them downward.  Also, individuals who have shoulder pain tend to have weakness of the lower trapezius muscles and limited thoracic extension.  The body gets the last 20 degrees of shoulder flexion range of motion from the thoracic spine.

I came across a study that compared the strength of the lower trapezius muscles in people who received Grade I joint mobilizations versus Grade IV joint mobilizations.

The origin of the lower trapezius is the spinous process of thoracic vertebrae 4/6 to 12.  The researchers focused on applying the mobilizations to those vertebrae for 30 seconds with a posterior to anterior force using the technique shown above at each spinous process.  They reported a significant increase in strength (6%) of the lower trapezius muscle for individuals who received Grade IV joint mobilizations to the thoracic spine.

It is important to incorporate joint mobility treatments to individuals who present with muscle weakness.

Reference:

Liebler EJ, Tufano-Coors L, Douris P, et al. The effects of thoracic spine mobilization on lower trapezius strength testing. J Man Manip Ther. 2001; 9(4):207-2012.